NUMA-aware garbage collection

ABSTRACT

Methods and systems for garbage collection are described. In some embodiments, Garbage collector threads may maximize local accesses and minimize remote access by copying Young objects and Old objects differently. When copying a Young object, a garbage collector thread may determine the lgroup of the pool that contains the object and copy the object to a pool of the same lgroup. The garbage collector thread may spread Old objects among lgroups by copying Old objects to pools of the same lgroup as the respective garbage collector thread. Additional methods and systems are disclosed.

INCORPORATION BY REFERENCE; DISCLAIMER

The following application is hereby incorporated by reference:application Ser. No. 13/077,619 filed on Mar. 31, 2011. The Applicanthereby rescinds any disclaimer of claim scope in the parentapplication(s) or the prosecution history thereof and advises the USPTOthat the claims in this application may be broader than any claim in theparent application(s).

FIELD OF THE INVENTION

This invention relates generally to memory management, and morespecifically to non-uniform memory access (NUMA) aware garbagecollection.

BACKGROUND

Memory is generally allocated for a program during execution from a poolof a memory area called a heap. Garbage collection is a form of memorymanagement for programs. During execution, the garbage collectorattempts to identify memory allocated to objects that are not in use bythe program, so that they may be deallocated (also referred to as“reclaimed”). An object may be in use by the program, or reachable, ifthe object can be accessed (also referred to as “reached”) by theprogram's current state. Since the precise allocation of space in theheap to objects is not known in advance, the memory allocated to objectscannot be accessed via the actual address of the memory. Rather, thememory may be accessed indirectly by the program utilizing references.An object is reachable if it is referenced by a local variable orparameter in a currently invoked function or a global variable, or isreferenced by a reachable object. The garbage collector deallocatesmemory allocated to objects that are no longer reachable. It must notdeallocate memory occupied by objects that are still reachable.

For non-uniform memory access (NUMA) computing devices, the cost ofmemory accesses by processing units (as well as hardware threads, cores,and so on) is not constant. Rather, in such computing devices, the costof memory access depends on whether the memory is local or remote to aparticular processing unit. In such computing devices, a first memorymay be classified as local to a particular processing unit and a secondclassified as remote when the particular processing unit is able toaccess the first memory faster than the second memory.

An “lgroup” (locality group) is a group of processing units (and/orhardware threads, cores, and so on) and memory in a NUMA computingdevice for which all memory accesses are local. Memory access from aprocessing unit in one lgroup to memory of another lgroup would resultin a remote, and hence slower, access. Lgroups may correspond to asingle processing unit socket and the m mory attached to it, multipleprocessing units and memories attached to a single printed circuit boardwhen the computing device includes multiple printed circuit boards eachwith one or more processing units and memories, multiple computingdevices arranged in a cloud computing configuration, and so on.

Maximization of local memory accesses (i.e., keeping memory accesseswithin an lgroup as much as is possible) and minimizing remote memoryaccesses may improve overall system performance and efficiency.Typically, approaches to maximizing local memory accesses and minimizingremote memory accesses in NUMA computing devices deal with optimizingaccesses by application threads. Overall system performance andefficiency may be improved by maximizing local memory accesses andminimizing remote memory accesses in NUMA computing devices for garbagecollector threads.

SUMMARY

In some embodiments, a young object is accessed from a source region ofa first pool by a garbage collector thread. The first pool is associatedwith a lgroup. A processing unit and a memory are in communication withand local to a processing unit being grouped in the lgroup. The firstpool is deployed within heap space of the memory. Space is allocated forthe young object in a survivor region associated with a second pool. Thesecond pool is associated with the same lgroup. The young object iscopied to the allocated space in the survivor region associated with thesecond pool.

In some embodiments, an object is accessed from a source region of afirst pool by a garbage collector thread. The first pool is associatedwith a lgroup among. A processing unit and a memory in communicationwith and local to the processing unit being grouped in the lgroup. Thefirst pool is deployed within heap space of the memory. The garbagecollector thread is associated with a second pool associated with adifferent lgroup. Space is allocated for the object in an old regionassociated with the second pool. The object is copied to allocated spacein the old region associated with the second pool.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the present disclosure. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate subject matter of the disclosure.Together, the descriptions and the drawings serve to explain theprinciples of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood and its numerousfeatures made apparent to those skilled in the art by referencing theaccompanying drawings.

FIG. 1 is a block diagram illustrating a first example system fornon-uniform memory access aware garbage collector thread processing, inaccordance with an embodiment of the present disclosure;

FIG. 2 is a block diagram illustrating a second example system fornon-uniform memory access aware garbage collector thread processing, inaccordance with an embodiment of the present disclosure;

FIG. 3 is a block diagram illustrating a third example system fornon-uniform memory access aware garbage collector thread processing, inaccordance with an embodiment of the present disclosure;

FIG. 4A is a method diagram illustrating a method for non-uniform memoryaccess aware garbage collector thread processing, which may be performedby the systems of FIG. 1, 2, 3 , and so on in accordance with anembodiment of the present disclosure;

FIG. 4B is a method diagram illustrating sub-operations for theoperation of the method of FIG. 4A of processing queues in the orderedsequence, in accordance with the present disclosure;

FIG. 5A is a method diagram illustrating a method for queuing objectsfor non-uniform memory access aware garbage collector thread processing,which may be performed by the systems of FIG. 1, 2, 3 , and so on inaccordance with the present disclosure;

FIG. 5B is a method diagram illustrating sub-operations for theoperation of the method of FIG. 5A of determining the lgroup of the poolthat contains an object, in accordance with the present disclosure;

FIG. 6A is a block diagram illustrating a structure of references in aheap space in an example implementation of the system of FIG. 1 ;

FIG. 6B is a block diagram illustrating the heap space of FIG. 6Adivided into a number of pools;

FIG. 6C is a block diagram illustrating an example lgroup table for thepools illustrated of FIG. 6B;

FIG. 6D is a block diagram illustrating garbage collector threadsexecuting on the example implementation of the system of FIG. 1 ;

FIGS. 6E(i) through 6E(x) are block diagrams illustrating an example ofthe garbage collector threads of FIG. 6D queuing the objects of the heapspace illustrated in FIGS. 6A and 6B according to a breadth-firstalgorithm and then processing the queues;

FIGS. 6F(i) through 6F(xii) are block diagrams illustrating an exampleof the garbage collector threads of FIG. 6D queuing the objects of theheap space illustrated in FIGS. 6A and 6B according to a depth-firstalgorithm and processing the queues according to the same ordering asFIGS. 6E(i) through 6E(x);

FIG. 6G(i) is a block diagram illustrating the heap space shown in FIG.6B with the pools divided into Young regions and Old regions;

FIG. 6G(ii) is a block diagram illustrating the heap space shown in FIG.6G(i) after the processing shown in FIGS. 6F(i) through 6F(xii) wherethe processing is a first example of a region based copying garbagecollection cycle;

FIG. 6G(iii) is a block diagram illustrating the heap space shown inFIG. 6G(i) after the processing shown in FIGS. 6F(i) through 6F(xii)where the processing is second example of a region based copying garbagecollection cycle;

FIG. 6G(iv) is a block diagram illustrating a more balanced version ofthe heap space shown in FIG. 6G(iii);

FIG. 7 is a block diagram illustrating a fourth example system fornon-uniform memory access aware garbage collector thread processing, inaccordance with an embodiment of the present disclosure;

FIG. 8 is a block diagram illustrating the regions of FIG. 7 , inaccordance with an embodiment of the present disclosure;

FIG. 9A is a method diagram illustrating a method for non-uniform memoryaccess aware garbage collector thread processing, which may be performedby the system of FIG. 8 , in accordance with an embodiment of thepresent disclosure;

FIG. 9B is a method diagram illustrating sub-operations for the methodof FIG. 9A of copying a Young object by determining an lgroup of acontaining pool and copying to a pool with same lgroup, in accordancewith the present disclosure;

FIG. 9C is a method diagram illustrating sub-operations of the method ofFIG. 9A of copying an Old object by copying to a pool with the samelgroup as the respective garbage collector thread, in accordance withthe present disclosure;

FIG. 10A is a block diagram illustrating a structure of references in aheap space in an example implementation of the system of FIG. 8 ;

FIG. 10B is a block diagram illustrating the heap space of FIG. 10Adivided into a number of pools with the pools divided into Young regionsand Old regions;

FIG. 10C is a block diagram illustrating the heap space shown in FIG.10B after a first example of one or more region based copying garbagecollection cycles;

FIG. 10D is a block diagram illustrating the heap space shown in FIG.10B after a second example of one or more region based copying garbagecollection cycles;

FIG. 11A is a block diagram illustrating thread structures incommunication with an allocation region in an example implementation ofthe system of FIG. 8 ;

FIG. 11B is a block diagram illustrating the thread structures and theallocation region of FIG. 11A where a portion of a thread localallocation buffer has been allocated;

FIG. 11C is a block diagram illustrating the thread structures and theallocation region of FIG. 11B where a new thread local allocation bufferis created; and

FIG. 11D is a block diagram illustrating the thread structures and theallocation region of FIG. 11C where a portion of the new thread localallocation buffer is allocated.

DETAILED DESCRIPTION

The present disclosure describes systems and methods for non-uniformmemory access aware garbage collector thread processing. In someimplementations, a heap space is divided into pools and garbagecollector threads include one or more separate queues for objectscontained in pools of local and remote lgroups. In such implementations,the garbage collector threads maximize local memory accesses andminimize remote memory accesses by ordered processing of the separatequeues. In various implementations, a heap space is divided into pools,which are further divided into regions. In such implementations, garbagecollector threads maximize local memory accesses and minimize remotememory accesses by copying Young objects to pools of the same lgroup asthe previous containing pool and Old objects to pools of the same lgroupas the respective garbage collector thread. In these ways, overallsystem performance and efficiency may be improved by maximizing localmemory accesses and minimizing remote memory accesses for the garbagecollector threads. In addition, copying Young objects to pools of thesame lgroup as the previous containing pool causes these objects to beretained in the same lgroup as the allocating thread.

FIG. 1 illustrates a first example of a system 100 for non-uniformmemory access aware garbage collector thread processing, in accordancewith an embodiment of the present disclosure. The system 100 has acomputing device 101 which includes a circuit board 102. The circuitboard 102 includes two processing units 103A and 103B which are coupledto two memories 104A and 104B.

The processing units 103A and 103B are both operable to access memories104A and 104B. However, in this first example, the processing unit 103Ais operable to access the memory 104A faster than the memory 104B andsimilarly the processing unit 103B is operable to access the memory 104Bfaster than the memory 104A. Thus, the memory 104A is defined as localto the processing unit 103A and remote to the processing unit 103B andthe memory 104B is defined as local to the processing unit 103B andremote to the processing unit 103A. As such, the processing unit 103Aand the memory 104A are grouped into a first lgroup and the processingunit 103B and the memory 104B are grouped into a second lgroup.

The processing units 103A and 103B are each configured to executeinstructions implementing any number of execution threads 105A and 105Bof possible programs as well as garbage collector threads 106A and 106B.Each garbage collector thread may be assigned to the same lgroup as therespective processing unit that executes the garbage collector thread.As illustrated, the memories 104A and 104B include a heap space 108A and108B for the programs related to the execution threads 105A and 105B.The heap spaces 108A and 108B are divided into a number of pools 111Aand 111B that are assigned to the same lgroup as the respective memorythat contains the pool.

The processing units 103A and 103B may process instructions that operateto maximize accesses to memories in the same lgroup as the respectiveprocessing unit and minimize accesses to memories in other lgroups toreduce latency and improve overall system performance and efficiency.The garbage collector threads 106A and 106B are each associated with anumber of queues 107A and 107B located in the memory 104A and 104Brespectively. As illustrated, the garbage collector threads 106A and106B are associated with a local queue Q1 for objects contained in poolsassigned to the same lgroup as the respective garbage collector threadand a remote queue Q2 for objects contained in pools assigned todifferent lgroups than the respective garbage collector thread, althoughin various implementations the garbage collector threads 106A and 106Bmay include a remote queue Q2 for each lgroup other than the lgroup ofthe respective garbage collector thread. Each garbage collector threadmay process the queues in an ordered sequence to maximize accesses tothe garbage collector threads lgroup and minimize accesses to otherlgroups. The processing of the queues may be part of one or more garbagecollection cycles, one or more marking cycles, and so on.

When a garbage collector thread queues an object, the garbage collectorthread may determine the lgroup of the pool that contains the object andadd the object to the local queue Q1 if the lgroup of the pool is thesame as the lgroup of the garbage collector thread or the remote queueQ2 if the lgroup of the pool is different than the lgroup of the garbagecollector thread. The garbage collector thread may determine the lgroupof the pool that contains the object by constructing and referencing anlgroup table. The lgroup table may be a coarse granularity table thatincludes an lgroup entry for each of the pools. The garbage collectorthread may reference the lgroup table by computing an index from theaddress of the object and mapping that index to the lgroup table.

For example, each pool may be 4000 kilobytes in size and the garbagecollector thread may compute an index from the address of an object bydividing the address by 4000 and discarding the remainder. In thisexample, an index of 0 may be computed from either an object in thefirst pool in the heap space with an address of 2048 or an object in thefirst pool with an address of 2304. Similarly, an index of 2 may becomputed from either an object in the third pool in the heap space withan address of 8192 or an object in the third pool in the heap space withan address of 8448. Although the present example presents a particularformula for generating indexes from the address of an object in a pool,it is understood that numerous formula involving numerous mathematicaloperations could be utilized to generate indexes from the addresses ofobjects such that the indexes are the same for objects in the same pooland different for objects in different pools.

Further, as illustrated, each memory 104A includes a Stack A 109A forthe execution threads executing on the processing unit A 103A and thememory 104B includes a Stack B 109B for the execution threads executingon the processing unit B 103B. When queuing objects, each garbagecollector thread may first scan root stacks of threads of processingunits of the same lgroup as the garbage collector thread before scanningroot stacks of execution threads executing on processing units assignedto lgroups different from the garbage collector thread. As executionthreads in the same lgroup as the garbage collector thread may referencemore objects contained in pools of the same lgroup as the garbagecollector thread than objects contained in pools of different lgroups,the garbage collector threads may thus queue more objects contained inpools of the same lgroup as the garbage collector thread than objectscontained in pools of other lgroups.

In some implementations, the garbage collector threads 106A and 106B maybe threads of a regionalized copying garbage collector that divides theheap space into at least one or more Young regions and one or more Oldregions. Each pool may contain one or more of the Young regions and/orone or more of the Old regions. When an object is allocated, the objectmay be termed “Young” and contained within a Young region. If the Youngobject has not been deallocated after a period of time, such as one ormore garbage collection cycles, the object may be copied to an Oldregion, promoting the object to the status of an “Old object.” In suchimplementations, in addition to ordered processing of the queues 106Aand 106B, the garbage collector threads 106A and 106B may maximize localaccesses and minimize remote accesses by copying Young objects and Oldobjects differently.

When copying a Young object, a garbage collector thread may determinethe lgroup of the pool that contains the object and copy the object to apool of the same lgroup. As most Young objects may be used primarily bythe execution thread that created the objects, Young objects will thuslybe kept local to the lgroup that may primarily utilize the objects.However, Old objects may not be primarily utilized by any particularexecution thread and thus the garbage collector threads may spread Oldobjects among the lgroups by copying Old objects to pools of the samelgroup as the respective garbage collector thread. In order to implementsuch copying, garbage collector threads may include pointers to freespace in one or more pools of each lgroup to indicate where the garbagecollector thread will copy Young objects and a pointer to free space inone or more pools of the same lgroup as the respective garbage collectorthread to indicate where the garbage collector thread will copy Oldobjects.

However, if one or more garbage collector threads of a particular lgroupcopy more Old objects than garbage collector threads of other lgroups,the Old objects may not be spread across the various lgroups. As such,the garbage collector threads may monitor the copying of Old objectsduring one or more garbage collection cycles to determine that the Oldobjects are not evenly spread across lgroups. If the garbage collectorthreads determine that the Old objects are not evenly spread acrosslgroups, the garbage collector threads may copy Old objects to pools ofvarious lgroups, such as in a round-robin fashion, rather than copyingOld objects to pools of the same lgroup as the garbage collector thread.

While the system 100 has been described as including computing device101, a circuit board 102, processing units 103A and 1038, and memories104A and 104B, it will be understood that system 100 is illustrative andthat one or more computing devices 101, which may be communicablycoupled; dual core, multiple core, parallel processing, and otherprocessing unit 103A and 103B configurations; and/or multiple memories104A and 104B and types of memory 104A and 104B such as random accessmemory, read-only memory, flash memory, cache memory, on-chip memory,off-chip memory, and etc. may be utilized without departing from thescope of the present disclosure.

By way of a first illustration, FIG. 2 is a second example of a system200 for non-uniform memory access aware garbage collector threadprocessing. The system 200 has a computing device 201 which includescommunicably coupled circuit boards 202A and 202B. The circuit board202A includes a processing unit A 203A communicably coupled to a memory204A and the circuit board 202B includes a processing unit B 203Bcommunicably coupled to a memory 204B. The processing units 203A and203B are both operable to access memories 204A and 204B. However, inthis second example, the processing unit 203A is operable to access thememory 204A faster than the memory 204B and similarly the processingunit 203B is operable to access the memory 204B faster than the memory204A. Thus, the memory 204A is defined as local to the processing unit203A and remote to the processing unit 203B and the memory 204B isdefined as local to the processing unit 203B and remote to theprocessing unit 203A. As such, the processing unit 203A and the memory204A are grouped into a first lgroup and the processing unit 203B andthe memory 204B are grouped into a second lgroup.

By way of a second illustration, FIG. 3 is a third example of a system300 for non-uniform memory access aware garbage collector threadprocessing. The system 300 includes a first computing device 301Acommunicably coupled through a network communication link to a secondcomputing device 301B. The first computing device 301A includes aprocessing unit A 303A communicably coupled to a memory 304A and thesecond computing device 301B includes a processing unit B 303Bcommunicably coupled to a memory 304B. The processing units 303A and303B are both operable to access memories 304A and 304B. However, inthis third example, the processing unit 303A is operable to access thememory 304A faster than the memory 304B and similarly the processingunit 303B is operable to access the memory 304B faster than the memory304A. Thus, the memory 304A is defined as local to the processing unit303A and remote to the processing unit 303B and the memory 304B isdefined as local to the processing unit 303B and remote to theprocessing unit 303A. As such, the processing unit 303A and the memory304A are grouped into a first lgroup and the processing unit 303B andthe memory 304B are grouped into a second lgroup.

Referring again to FIG. 1 , the computing device 101 may also includeone or more tangible machine-readable storage media 112, communicablycoupled to the processing units 103A and 103B and/or the memories 104Aand 104B, which may include, but are not limited to, magnetic storagemedia such as a floppy diskette; optical storage media such as a CD-ROM;magneto-optical storage media; read only memory; random access memory;erasable programmable memory such as EPROM or EEPROM; flash memory; orother types of media suitable for storing electronic information. Thecomputing device 101 may also include, not shown, one or more inputand/or output devices such as displays, keyboards, mice, printers,scanners, and so forth; one or more buses such as universal serial bus,small computer system interface, and so forth; and/or one or morecommunication components such as modems, Ethernet adapters, wirelessEthernet adapters, and so forth. The instructions the processing units103A and 103B execute to implement the execution threads 105A and 105Band the threads of a garbage collector 106A and 106B may be stored inthe memories 104A and 104B and/or the storage media 112.

FIG. 4A illustrates a method 400 for non-uniform memory access awaregarbage collector thread processing, which may be performed by thesystem 100, in accordance with an embodiment of the present disclosure.Although method 400 will be described below as performed by the system100, the method 400 may be performed by the system 200, the system 300,or other computer systems without departing from the scope of thepresent disclosure. The method 400 is a computer-implemented method, andas such is performed by a processing unit, such as the processing units103A and 103B, executing one or more sets of instructions included in acomputer program product stored in a machine-readable medium, such asthe memories 104A and 104B.

An operating system of the computing device 101 assigns processing unitsand memories into lgroups based on access latency between each of theprocessing units and the memories. A processing unit may be assigned tothe same lgroup as a memory if the access latency between thatprocessing unit and that memory is less than between that processingunit and other memories.

In an operation 420, the computing device 101 divides a heap space intoa number of pools that are each contained in one of the memories. Thecomputing device 101 assigns each of the pools to the same lgroup as thememory that contains the respective pool. Each of the pools may be thesame size, such as 1024 kilobytes.

In an operation 430, the computing device 101 provides a number ofgarbage collector threads executing on one or more processing units,each garbage collector thread including a number of queues. Thecomputing device 101 assigns each garbage collector thread to the samelgroup as the processing unit that executes the respective garbagecollector thread. The number of queues for each garbage collector threadincludes at least a local queue for objects contained in pools of thesame lgroup as the respective garbage collector thread and at least oneremote queue for objects contained in pools of different lgroups fromthe respective garbage collector threads. In some implementations, thenumber of queues may include a separate remote queue for each lgroupother than the lgroup of the respective garbage collector thread.

In an operation 440, the computing device 101 executes the garbagecollector threads to process the queues in an ordered sequence tomaximize accesses to the respective garbage collector thread's lgroupand minimize accesses to other lgroups.

FIG. 4B illustrates sub-operations that may be involved in processingthe queues in the ordered sequence. In a sub-operation 440A, a garbagecollector thread may process the local queue. In a sub-operation 440B,the garbage collector thread may process the queues of other garbagecollector threads that are for objects stored in pools of the samelgroup as the garbage collector thread. In a sub-operation 440C, thegarbage collector thread may process the remote queue(s). In asub-operation 440D, the garbage collector thread may process the queuesof other garbage collector threads that are for objects stored in poolsof lgroups different than the lgroup of the garbage collector thread. Byprocessing the queues in this ordered sequence, each garbage collectorthread is biased to process objects contained in pools of the samelgroup as the respective garbage collector thread as much as possiblewithout being idle.

FIG. 5A illustrates a method 500 for queuing objects for non-uniformmemory access aware garbage collector thread processing, which may beperformed by the system 100, in accordance with an embodiment of thepresent disclosure, Although method 500 will be described below asperformed by the system 100, the method 500 may be performed by thesystem 200, the system 300, or other computer systems without departingfrom the scope of the present disclosure. The method 500 is acomputer-implemented method, and as such is performed by a processingunit, such as the processing units 103A and 103B, executing one or moresets of instructions included in a computer program product stored in amachine-readable medium, such as the memories 104A and 104B.

In an operation 510, a garbage collector thread determines the lgroup ofthe pool that contains an object. In an operation 520, the garbagecollector thread adds the object to the local queue if the lgroup of thepool is the same lgroup as that of the garbage collector thread. In anoperation 530, the garbage collector thread adds the object to theremote queue(s) if the lgroup of the pool is different than the lgroupof the garbage collector thread.

FIG. 5B illustrates sub-operations that may be involved in determiningthe lgroup of the pool that contains the object. In a sub-operation510A, the computing device 101 constructs an lgroup table that includesentries specifying the lgroup of each of the pools. In a sub-operation510B, the garbage collector thread computes an index to the lgroup tablefrom the address of the object. In a sub-operation 510C, garbagecollector thread maps the index to an entry in the lgroup table.

FIG. 6A is a block diagram illustrating the structure of references in aheap space in an example implementation of the system 100. Asillustrated, the heap space includes references from a root set ofreferences in the Stack A 109A and references from a root set ofreferences in the Stack B 109B. The root sets of references may includeexternal references from anywhere in the respective call stack, localvariables and parameters in functions currently being invoked, and anyglobal variables. As illustrated, the heap includes memory that has beenallocated to objects A through Z.

The root set of references in the Stack A 109A includes externalreferences to objects A, B, and C. The root set of references in theStack B 109B includes external references to objects L and M. Object Areferences objects D and E. Object E references objects J and K. ObjectB references objects F, G, and H. Object C references object I. Object Lreferences objects N, O, and P. Object N references objects T, U, and V.Object P references objects W and X. Object M references objects Q, R,and S. Object Q references object Y. Object S references object Z. Thus,in this example, all objects A through Z are currently reachable and noobjects are currently unreachable.

FIG. 6B is a block diagram illustrating the heap space of FIG. 6Adivided into a number of pools contained in the memories 104A and 104B.As illustrated, pools 1, 2, and 3 are contained in memory 104A. Asmemory 104A is assigned to lgroup 1 in this example, pools 1, 2, and 3are also assigned to lgroup 1. Also as illustrated, pools 4, 5, and 6are contained in memory 104B. As memory 104B is assigned to lgroup 2 inthis example, pools 4, 5, and 6 are also assigned to lgroup 2. ObjectsA, C, D, E, L, and M are currently contained in pool 1. Objects G, H, T,U, V, W, and X are currently contained in pool 2. Objects B, F, J, K, Y,and Z are currently stored in pool 4. Objects I, N, O, P, Q, R, and Sare currently stored in pool 6. No objects are currently stored in pools3 and 5.

FIG. 6C is an example lgroup table for the pools illustrated in FIG. 6B.As illustrated, the lgroup table includes an entry for each of the pools1 through 6 that specifies the lgroup of the respective pool. As furtherillustrated, the entry for each pool includes an index. An indexcomputed from the address of any object contained in pool 1 may be 0.Similarly, an index computed from the address of any object contained inpool 2 may be 1, an index computed from the address of any objectcontained in pool 3 may be 2, and so on. Thus, the index computed fromthe address of an object in one of the pools may be mapped to the lgrouptable to determine the lgroup of the pool that contains the object.

FIG. 6D is a block diagram illustrating three garbage collector threadsA1, A2, and B1 executing on the example implementation of the system100. As illustrated, garbage collector threads A1 and A2 have beenassigned to lgroup 1 and garbage collector thread B1 has been assignedto lgroup 2. Also as illustrated, each garbage collector thread includesa local queue Q1 for objects contained in pools of the same lgroup asthe respective garbage collector thread and a remote queue Q2 forobjects contained in pools of lgroups different than the respectivegarbage collector thread.

FIGS. 6E(i) through 6E(x) illustrate an example of the three garbagecollector threads of FIG. 6D queuing the objects of the heap spaceillustrated in FIGS. 6A and 6B according to a breadth-first algorithmand then processing the queues. The queuing of the objects is performedaccording to a breadth-first algorithm as all references into the heapspace from the root set of references are queued before processing thequeues. In this example, garbage collector thread A1 is operable tobegin scanning for references into the heap from the beginning of theroot set of Stack A, garbage collector thread A2 is operable to beginscanning for references into the heap from the middle of the root set ofStack A, and garbage collector thread B1 is operable to begin scanningfor references into the heap from the beginning of the root set of StackB.

Thus, as illustrated in FIG. 6E(i), garbage collector thread A1 queuesobject A, garbage collector thread A2 queues objects B and C, andgarbage collector thread B1 queues objects L and M. Garbage collectorthread A1 queues object A in the local queue Q1 because object A iscontained in pool 1, which is assigned to the same lgroup as garbagecollector thread A1. Garbage collector thread A2 queues object B in theremote queue Q2 because object B is contained in pool 4, which isassigned to a different lgroup than the garbage collector thread A2.Garbage collector thread A2 queues object C in the local queue Q1because object C is contained in pool 1, which is assigned to the samelgroup as garbage collector thread A2. Garbage collector thread B1queues objects L and M in the remote queue Q2 because objects L and Mare contained in pool 1, which is assigned to a different lgroup thanthe garbage collector thread B1.

After queuing all references into the heap space from the root set ofreferences, the garbage collector threads A1, A2, and B1 perform theordered processing of the queues. As garbage collector thread A1includes object A in the local queue Q1, garbage collector thread A1will first process object A. As garbage collector thread A2 includesobject C in the local queue Q1, garbage collector thread A2 will firstprocess object C. Garbage collector thread B1 does not include anyobjects in the local queue Q1 and will thus process object B from theremote queue Q2 of garbage collector A2 as that queue is for objectsstored in pools assigned to the same group as garbage collector threadB1.

FIG. 6E(ii) illustrates garbage collector thread A1 processing object A,garbage collector thread A2 processing object C, and garbage collectorthread B1 processing object B. In processing these objects, garbagecollector thread A1 queues objects D and E which are referenced byobject A, garbage collector thread A2 queues object I which isreferenced by object C, and garbage collector thread B1 queues objectsF, G, and H which are referenced by object B. As objects D and E arecontained in a pool of the same lgroup as the garbage collector threadA1, garbage collector thread A1 queues objects D and E in the localqueue Q1. As object I is contained in a pool of the an lgroup differentthan the garbage collector thread A2, garbage collector thread A2 queuesobject I in the remote queue Q2. As object F is contained in a pool ofthe same lgroup as garbage collector thread B1 and objects G and H arecontained in pools of another lgroup, garbage collector thread B1 queuesobject F in the local queue Q1 and objects G and H in the remote queueQ2.

Next, garbage collector thread A1 selects object D from the local queueQ1 for processing and garbage collector thread B1 selects object F fromthe local queue Q2 for processing. Garbage collector thread A2 does notinclude any objects in the local queue Q1 and will thus process object Lfrom the remote queue Q2 of garbage collector B1 as that queue is forobjects stored in pools assigned to the same lgroup as garbage collectorthread A2. FIG. 6E(iii) illustrates garbage collector thread A1processing object D, garbage collector thread A2 processing object L,and garbage collector thread B1 processing object F. In processingobject L, garbage collector thread A2 queues objects N, O, and P in theremote queue Q2 as these objects referenced by object L are contained ina pool of a different lgroup than garbage collector thread A2.

Garbage collector thread A1 then selects object E from the local queueQ1 for processing. Garbage collector thread A2 and garbage collectorthread B1 do not include any objects in their local queues Q1 and thusgarbage collector thread A2 will process object M from the remote queueQ2 of garbage collector thread B1 and garbage collector thread B1 willprocess object I from the remote queue Q2 of garbage collector threadA2. As shown in FIG. 6E(iv), in processing these objects garbagecollector thread A1 queues objects J and K in the remote queue Q2 andgarbage collector thread A2 queues objects Q, R, and S in the remotequeue Q2.

Then, as none of the illustrated garbage collector thread include anobject in their local queues Q1, garbage collector thread A1 selectsobject G from the remote queue Q2 of garbage collector thread B1,garbage collector thread A2 selects object H from the remote queue ofgarbage collector thread B1, and garbage collector thread B1 selectsobject N from the remote queue Q2 of garbage collector thread A2. FIG.6E(v) illustrates that in processing object N, garbage collector threadB1 queues objects T, U, and V in the remote queue Q2.

Subsequently, as none of the illustrated garbage collector threadinclude an object in their local queues Q1, garbage collector thread A1selects object T from the remote queue Q2 of garbage collector threadB1, garbage collector thread A2 selects object U from the remote queueof garbage collector thread B1, and garbage collector thread B1 selectsobject O from the remote queue Q2 of garbage collector thread A2. Thisis illustrated in FIG. 6E(vi).

Next, as garbage collector thread A1 does not include any objects in itslocal queue Q1, garbage collector thread A1 selects object V from theremote queue Q2 of garbage collector thread B1. As garbage collectorthread A2 does not include any objects in its local queue Q1 and thereare no objects in queues of other garbage collector threads that are ofthe same lgroup as garbage collector thread A2, garbage collector threadA2 selects object P from its own remote queue Q2 for processing. Asgarbage collector garbage collector thread B1 does not include anyobjects in its local queue Q1, garbage collector garbage collectorthread B1 selects object Q from the remote queue Q2 of garbage collectorthread A2. FIG. 6E(vii) illustrates that, in processing these objects,garbage collector thread A2 queues objects W and X in its local queue Q1and garbage collector thread B1 queues object Y in its local queue Q1.

Following the processing of objects V, P, and Q, garbage collectorthread A2 selects object W and garbage collector B1 selects object Yfrom their respective local queues Q1 for processing. As garbagecollector thread A1 does not include any objects in its local queue Q1,garbage collector thread A1 selects object X from the local queue Q1 ofgarbage collector thread A2. This is illustrated in FIG. 6E(viii). Next,garbage collector thread A1 selects object J from its own remote queueQ2 as its own local queue Q1 is empty and there are no objects in queuesof other garbage collector threads that are contained in pools of thesame lgroup as garbage collector thread A1. Similarly, garbage collectorthread A2 selects object R from its own remote queue Q2 as its own localqueue Q1 is empty and there are no objects in queues of other garbagecollector threads that are contained in pools of the same lgroup asgarbage collector thread A2. As its own local queue Q1 is empty, garbagecollector thread B1 selects object S from the remote queue Q2 of garbagecollector thread A2. As illustrated in FIG. 6E(ix), in processing objectS, garbage collector thread B1 queues object Z in its local queue Q1.

Finally, garbage collector thread A1 selects object K from its ownremote queue Q2 as its own local queue Q1 is empty and there are noobjects in queues of other garbage collector threads that are containedin pools of the same lgroup as garbage collector thread A1. Garbagecollector thread B1 selects object Z from its local queue Q1. As noobjects remain in any queues, garbage collector thread A2 is idle. Thisis illustrated in FIG. 6E(x). Thus, all objects have been processed.

FIGS. 6F(i) through 6F(xii) illustrate an example of the three garbagecollector threads of FIG. 6D queuing the objects of the heap spaceillustrated in FIGS. 6A and 6B according to a depth-first algorithm andprocessing the queues according to the same ordering as FIGS. 6E(i)through 6E(x). The queuing of the objects is performed according to adepth-first algorithm as each garbage collection thread queues areference into the heap space from the root set of references and thenprocesses the queues before queuing the next reference into the heapspace from the root set of references.

It is understood that although FIGS. 6E(i) through 6E(x) illustratequeuing objects of the heap space illustrated in FIGS. 6A and 6Daccording to a breadth-first algorithm and FIGS. 6F(i) through 6F(xii)illustrate queuing objects of the heap space illustrated in FIGS. 6A and6D according to a depth-first algorithm, other queuing algorithms arepossible without departing from the scope of the present disclosure. Forexample, in a hybrid approach that combines breadth-first and depthfirst, a certain number of references into the heap from the root setmay be queued, such as 10. The queues may then be processed beforequeuing additional references into the heap from the root set.

The processing of objects of the heap space illustrated in FIGS. 6F(i)through 6F(xii) may be performed as part of a garbage collection cycleof a region-based copying garbage collector. FIG. 6G(i) illustrates anexample of the heap space shown in FIG. 6B with the pools divided intoYoung regions and Old regions. While FIG. 6G(i) shows each pool with asingle Young region and a single Old region, a pool can contain multipleYoung regions and/or multiple Old regions. A pool need not have bothtypes of regions or the same number of regions. In addition, the poolsof a heap space need not have the number and/or types of regions. Forexample, one pool could have one or multiple Young regions, another poolcould have one or multiple Old regions, and yet another pool could haveone or multiple of both Young and Old regions.

Objects shown as contained in a Young region are defined as Youngobjects and objects shown as contained in Old regions are defined as Oldobjects. In this example, through the processing of objects of the heapspace illustrated in FIGS. 6F(i) through 6F(xii) copies objects fromregions in pools 1, 2, 4, and 6 to regions in pools 3 and 5 beforereclaiming the regions in pools 1, 2, 4, and 6.

FIG. 6G(ii) shows the heap space of FIG. 6G(i) after the processingillustrated in FIGS. 6F(i) through 6F(xii) where all Young objects arecopied to Young regions in pools of the same lgroup as the pool thatpreviously contained the Young object and all Old objects are copied topools of the same lgroup as the garbage collector thread that processesthe object. However, this results in far more Old objects beingcontained in pools in lgroup 1 than are contained in pools in lgroup 2.

FIG. 6G(iii) shows the heap space of FIG. 6G(i) after the processingillustrated in FIGS. 6F(i) through 6F(xii). All Young objects have beencopied to Young regions in pools of the same lgroup as the pool thatpreviously contained the Young object and Old objects have been copiedto pools of the same lgroup as the garbage collector thread thatprocesses the object. When monitoring of copying determines that the Oldobjects are not being evenly spread across lgroups, Old objects may becopied to pools of lgroups in a round robin fashion to balance theobjects. FIG. 6G(iv) shows a more balanced version of FIG. 6G(iii) wheresome of the objects contained in the Old region of pool 3 have beenmoved to the Old region of pool 5.

FIG. 7 illustrates a fourth example of a system 800 for non-uniformmemory access aware garbage collector thread processing, in accordancewith an embodiment of the present disclosure. The system 800 has acomputing device 801 which includes a circuit board 802. The circuitboard 802 includes two processing units 803A and 803B which are coupledto two memories 804A and 804B.

The processing units 803A and 803B are both operable to access memories804A and 804B. However, in this fourth example, the processing unit 803Ais operable to access the memory 804A faster than the memory 804B andsimilarly the processing unit 803B is operable to access the memory 804Bfaster than the memory 804A. Thus, the memory 804A is defined as localto the processing unit 803A and remote to the processing unit 803B andthe memory 804B is defined as local to the processing unit 803B andremote to the processing unit 803A. As such, the processing unit 803Aand the memory 804A are grouped into a first lgroup and the processingunit 803B and the memory 804B are grouped into a second lgroup.

The processing units 803A and 803B are each configured to executeinstructions implementing any number of execution threads 805A and 805Bof possible programs. As illustrated, the memories 804A and 804B includea heap space 808A and 808B for the programs related to the executionthreads 805A and 805B. The heap space 808A and 808B is divided into anumber of pools 809A and 809B that are assigned to the same lgroup asthe respective memory that contains the pool. The processing units 803Aand 803B are also each configured to execute instructions implementingany number of garbage collector threads 806A and 806B of a regionalcopying garbage collector that divides the pools 809A and 809B of theheap space 808A and 808B into a number of regions 810A and 810B. Theregions 810A and 810B include at least one or more Young regions and oneor more Old regions. Each garbage collector thread may be assigned tothe same lgroup as the respective processing unit that executes thegarbage collector thread.

The processing units 803A and 803B operate to maximize accesses tomemories in the same lgroup as the respective processing unit andminimize accesses to memories in other lgroups to reduce latency andimprove overall system performance and efficiency. When an object isallocated, the object may be termed “Young” and contained within a Youngregion. If the Young object has not been deallocated after a period oftime, such as one or more garbage collection cycles, the object may becopied to an Old region, promoting the object to the status of an “Oldobject.”

The garbage collector threads 806A and 806B may maximize local accessesand minimize remote access by copying Young objects and Old objectsdifferently. When copying a Young object, a garbage collector thread maydetermine the lgroup of the pool that contains the object and copy theobject to a pool of the same lgroup. As most Young objects may be usedprimarily by the execution thread that created the objects, Youngobjects will thusly be kept local to the lgroup that may primarilyutilize the objects. However, Old objects may not be primarily utilizedby any particular execution thread and thus the garbage collectorthreads may spread Old objects among the lgroups by copying Old objectsto pools of the same lgroup as the respective garbage collector thread.In order to implement such copying, garbage collector threads mayinclude pointers to free space in one or more pools of each lgroup toindicate where the garbage collector thread will copy Young objects anda pointer to free space in one or more pools of the same lgroup as therespective garbage collector thread to indicate where the garbagecollector thread will copy Old objects.

However, if one or more garbage collector threads of a particular lgroupcopy more Old objects than garbage collector threads of other lgroups,the Old objects may not be spread across the various lgroups. As such,the garbage collector threads may monitor the copying of Old objectsduring one or more garbage collection cycles to determine that the Oldobjects are not evenly spread across lgroups. If the garbage collectorthreads determine that the Old objects are not evenly spread acrosslgroups, the garbage collector threads may copy Old objects to pools ofvarious lgroups, such as in a round-robin fashion, rather than copyingOld objects to pools of the same lgroup as the garbage collector thread.

FIG. 8 is a block diagram illustrating various regions that may be usedwith FIG. 7 . In general, new objects are allocated in the young regionand, when they survive for a certain period of time, are promoted ormoved as long-lived objects to an old region 812. A pool may include ayoung region, the old region 812, or both the young region and the oldregion 812. The young region of a pool is either an allocation region816 or a survivor region 814. The allocation region 816 is used byexecution threads 805A, 805B to reserve memory for most newly createdobjects. The survivor region 814 is used by garbage collector threads806A, 806B as storage in which the garbage collector threads 806A, 806Bsave the live but short-lived objects. Typically, objects that survivetheir first garbage collection are moved from the allocation region 816to the survivor region 814.

FIG. 9A illustrates a method 900 for non-uniform memory access awaregarbage collector thread processing, which may be performed by thesystem 800, in accordance with an embodiment of the present disclosure.The method 900 is a computer-implemented method, and as such isperformed by a processing unit, such as the processing units 803A and803B, executing one or more sets of instructions included in a computerprogram product stored in a machine-readable medium, such as thememories 804A and 804B.

The computing device 801 assigns processing units (803A, 803B) andmemories (804A, 804B) into lgroups based on access latency between eachof the processing units 803A, 803B and the memories 804A, 804B. Aprocessing unit may be assigned to the same lgroup as a memory if theaccess latency between that processing unit and that memory is less thanbetween that processing unit and other memories.

In an operation 920, the computing device 801 divides a heap space(808A, 808B) into a number of pools (809A, 809B) that are each containedin one of the memories 804A, 804B and divides the number of pools 809A,809B into at least a number of Young Regions and Old Regions. Thecomputing device 801 assigns each of the pools 809A, 809B to the samelgroup as the memory 804A, 804B that contains the respective pool. Eachof the pools 809A, 809B may be the same size, such as 256 kilobytes.

In an operation 930, the computing device 801 utilizes a garbagecollector thread (806A or 806B) to copy a Young object as part of one ormore garbage collections. As part of copying the Young object, thegarbage collector thread determines the lgroup of the pool thatpreviously contained the Young object and copies the Young object to apool of the same lgroup. In an operation 940, the computing device 801utilizes a garbage collector thread to copy an Old object as part of thegarbage collection. As part of copying the Old object, the garbagecollector thread copies the Old object to a pool of the same lgroup asthe garbage collector thread.

The method 900 may also include operations 950 through 970. In anoperation 950, the computing device 801 utilizes the garbage collectorthread to monitor the copying of objects during the garbage collection.The garbage collector thread may monitor the copying of Old objectsand/or Young objects. In an operation 960, the garbage collector threaddetermines, based on the monitoring, that objects in the heap space arenot evenly spread across lgroups. This determination may be based on Oldobjects and/or Young objects. In an operation 970, when the garbagecollector thread copies Old objects, the garbage collector thread copiesold objects to pools of the various lgroups in a round-robin fashion.

The copying of objects described in the above operations 930, 940 maymove the objects between regions. The operation 930 may copy a Youngobject from the allocation region 816 to the survivor region 814. Theoperation 940 may copy a Young object from the survivor region 814 tothe Old region, thereby re-designating the Young object as an Oldobject. Certain lgroup affinities, or association of objects withregions of certain lgroups, may be maintained during the operations 930,940.

FIG. 9B illustrates sub-operations that may be involved in copying aYoung object to a pool with the same lgroup by determining the lgroup ofthe object's containing pool. In a sub-operation 930A, the garbagecollector thread accesses a Young object from a source region of a firstpool. The source region may be an allocation region or a source survivorregion. The Young object may be accessed as part of a first garbagecollection process or during a subsequent garbage collection process.The first pool may be associated with a particular lgroup among a numberof lgroups. The particular lgroup includes a particular processing unitand a particular memory in communication with and local to theparticular processing unit.

In a sub-operation 930B, the garbage collector thread determines theparticular lgroup associated with the first pool. In a sub-operation930C, the garbage collector thread may select the survivor region 814associated with the second pool based on a determination that theparticular lgroup is associated with the first pool. In a sub-operation930D, the garbage collector thread allocates space for the Young objectin the survivor region 814 associated with a second pool associated withthe same particular lgroup. The allocation of space may then be based onselection of the survivor region 814. In a sub-operation 930E, thegarbage collector thread copies the Young object to allocated space inthe survivor region 814 associated with the second pool. The Youngobject may thus be copied from the allocation region or the sourcesurvivor region to the survivor region 814. The Young object may becopied multiple times using the foregoing sub-operations while stayingwithin survivor regions of the same lgroup.

FIG. 9C illustrates sub-operations that may be involved in copying anOld object to pool with the same lgroup as the respective garbagecollector thread. In a sub-operation 940A, the garbage collector threadaccesses an object from a source region of a first pool. The first poolmay be associated with a particular lgroup among a number of lgroups.The source region may be an allocation region, a source survivor region,or a source old region. The garbage collection thread may be associatedwith a second pool associated with a different lgroup than theparticular lgroup.

In some embodiments, the accessed object is primarily being manipulatedby the execution thread that created the object. In other embodiments,the object is being manipulated by a different execution thread than theexecution thread created the object.

In general, objects are accessed from the pool with which the garbagecollector thread is associated. However, if a determination is made thatthere are no further objects to access in the associated pool, the Youngobject may then be accessed from the source region of a different pool.For example, a Young object may be accessed from an allocation region ofa pool associated with a first lgroup by a garbage collection threadexecuting on a different processing unit and associated with a second,different lgroup.

In a sub-operation 940B, the garbage collector thread determines thatthe particular lgroup is associated with the first pool. In asub-operation 940C, the garbage collector thread selects the Old regionassociated with the second pool based on a determination that theparticular lgroup is associated with the first pool. In a sub-operation940D, the garbage collector thread allocates space for the object in anOld region associated with the second pool. The allocation of space maybe based on selection of the Old region. In a sub-operation 940E, thegarbage collector thread copies the object to allocated space in the Oldregion associated with the second pool. The object may thus be copiedfrom the allocation region, the source survivor region, or the sourceOld region to the Old region associated with the second pool.

In some embodiments, the operations performed in the FIGS. 9B and 9C maymaximize the efficiency of the execution threads 805A, 805B. By moreevenly distributing objects among processing units 803A, 803B hot spotsor areas of concentrated processing on the processing unit 803A, 803Bthat would otherwise interfere with execution efficiency may be avoided.

FIG. 10A is a block diagram illustrating the structure of references ina heap space in an example implementation of the system 800. Asillustrated, the heap space includes references from a root set ofreferences. The root sets of references may include external referencesfrom anywhere in the call stack, local variables and parameters infunctions currently being invoked, and any global variables. Asillustrated, the heap includes memory that has been allocated to objectsa through z. The root set of references includes external references toobjects a, b, c, l, and m. Object a references objects d and e. Object ereferences objects j and k. Object b references objects f, g, and h.Object c references object i. Object l references objects n, o, and p.Object n references objects t, u, and v. Object p references objects wand x. Object m references objects q, r, and s. Object q referencesobject y. Object s references object z. Thus, in this example, allobjects a through z are currently reachable and no objects are currentlyunreachable.

FIG. 10B is a block diagram illustrating the heap space of FIG. 10Adivided into a number of pools 809A and 809B contained in the memories804A and 804B with the pools divided into Young regions and Old regions.Objects shown as contained in a Young region are defined as Youngobjects and objects shown as contained in Old regions are defined as Oldobjects. As illustrated, pools 1, 2, and 3 are contained in memory 804A.As memory 804A is assigned to lgroup 1 in this example, pools 1, 2, and3 are also assigned to lgroup 1. Also as illustrated, pools 4, 5, and 6are contained in memory 804B. As memory 804B is assigned to lgroup 2 inthis example, pools 4, 5, and 6 are also assigned to lgroup 2.

Young objects a, c, and d are currently contained in a Young region inpool 1. Old objects e, l, and m are currently contained in an Old regionin pool 1. Young objects g, h, t, u, v, and w are currently contained ina Young region in pool 2. Old object x is currently contained in an Oldregion in pool 2. Young objects b, f, j, and k are currently containedin a Young region in pool 4. Old objects y and z are currently containedin an Old region in pool 4. Young objects l, n, o, s, and r arecurrently contained in a Young region in pool 6. Old objects p and q arecurrently contained in an Old region in pool 6. No objects are currentlystored in pools 3 and 5.

FIG. 10C is a block diagram illustrating the heap space of FIG. 10Bafter one or more garbage collection cycles that results in copying ofobjects from regions in pools 1, 2, 4, and 6 to regions in pools 3 and 5in accordance with operations 910 through 940. As shown, all Youngobjects were copied to Young regions in pools of the same lgroup as thepool that previously contained the Young object. As further shown, Oldobjects z, y, x, q, p, and m were copied by one or more garbagecollector threads of lgroup 1 and Old objects l and e were copied by oneor more garbage collector threads of lgroup 2. Thus, Old objects z, y,x, q, p, and m were copied to an Old region in pool 3 and Old objects land e were copied to an Old region in pool 5. However, this results infar more Old objects being contained in pools in lgroup 1 than arecontained in pools in lgroup 2.

FIG. 10D is a block diagram illustrating the heap space of FIG. 10Bafter one or more garbage collection cycles that results in copying ofobjects from regions in pools 1, 2, 4, and 6 to regions in pools 3 and 5in accordance with operations 910 through 970. As shown, all Youngobjects are copied to Young regions in pools of the same lgroup as thepool that previously contained the Young object. Also as shown, Oldobjects were copied to pools of the same lgroup as the garbage collectorthread that processed the object until monitoring of copying determinedthat the Old objects are not being evenly spread across lgroups. Then,Old objects were copied to pools of lgroups in a round robin fashion.Thus, the Old objects contained in pools more evenly spread acrosslgroup 1 and lgroup 2.

When objects are allocated or moved to various regions, there is thepotential for multiple threads to seek to manipulate the same region orobject space at the same time and thus cause a programming error. Oneoption to avoid a programming error is to lock the entire region whileone thread is accessing the region thereby preventing other threads frommanipulating the same space. However, this option may cause a delay ascertain threads wait for others threads to finish before starting theiroperations. Another option that avoids frequent use of locks and theaforementioned programming error includes splitting each region intolocal allocation buffers that are individually associated with threads.Since these portions of the regions are only manipulated by theirrespective threads, locks are not needed when multiple threads accessthe same region at the same time.

FIG. 11A is a block diagram illustrating thread structures incommunication with the allocation region 816 in an exampleimplementation of the system 800. As illustrated, the allocation region816 includes allocation sub-regions 1102-1114. An unallocated sub-region1102 not associated with a thread local allocation buffer (TLAB) isshown. In general, a TLAB is a portion of a region that is used forallocation by a single thread. The use of a TLAB enables the thread toperform faster object allocation than by use of an atomic operation on atop pointer that is shared across multiple threads. A TLAB 1 includes anallocated sub-region 1104 and an unallocated sub-region 1106. Anunallocated sub-region 1108 not associated with a TLAB is further shown,followed by an allocated sub-region 1110 and an unallocated sub-region1112 of TLAB 2. A further unallocated sub-region 1114 is shown.Unallocated sub-regions of the allocation region 816 may be used tospace out allocated sub-regions (e.g., to avoid hotspots) or to beavailable for further TLABs.

Two thread structures 1116, 1118 are shown to have pointers into theallocation region 816. A thread structure 1 includes a top pointer 1120to the end of the allocated sub-region 1104 of TLAB 1 and an end pointer1122 to the end of the unallocated sub-region 1106 of TLAB 1. A threadstructure 2 includes a top pointer 1124 to the end of the allocatedsub-region 1110 of TLAB 2 and an end pointer 1126 to the end of theunallocated sub-region 1112 of TLAB 2.

FIG. 11B is a block diagram illustrating the thread structures and theallocation region 816 of FIG. 11A where a portion of a TLAB has beenallocated. As illustrated, the unallocated sub-region 1106 of FIG. 11Ahas been allocated without the use of an allocation lock. As such,allocated sub-region 1104 has grown to include the unallocatedsub-region 1106 of FIG. 11A. The top pointer 1120 has been incrementedand thus is shown to now point to the end of the allocated region 1104.Thus, the top pointer 1120 and the end pointer 1122 are now pointing tothe same position. TLAB 1 is now full and as such, no further objectscan presently be created in TLAB 1.

In general, the allocation lock is used to prevent multiple threads fromtrying to access or otherwise manipulate the same space in theallocation region at the same time. Since each TLAB is exclusivelyassociated with a particular thread, there is no need for an allocationlock when writing to the TLAB. The allocation lock may be used, however,when creating a new TLAB and associating the unallocated space of theTLAB with a particular thread.

FIG. 11C is a block diagram illustrating the thread structures and theallocation region 816 of FIG. 11A where a new TLAB is created. Asillustrated, the un-allocated sub-region 1114 is divided intounallocated sub-regions 1128, 1130, and 1132. The new unallocatedsub-region 1130 becomes TLAB 3 and is associated with thread structure1. The top pointer 1120 points to the beginning of the unallocatedsub-region 1130 and the end pointer 1122 points to the end of theunallocated sub-region 1130. As such, additional objects can be createdby the thread associated with thread structure 1 in the unallocatedsub-region 1130 without the need for an allocation lock.

FIG. 11D is a block diagram illustrating the thread structures and theallocation region of FIG. 11A where a portion of the new TLAB isallocated. As illustrated, an allocated sub-region 1134 is allocatedfrom a portion of the unallocated sub-region 1130 of the TLAB 3. The toppointer 1120 is incremented so that the pointer points at the beginningof the unallocated sub-region 1132.

As described above, the TLABs may be used to limit the number ofallocation locks used when creating new objects in the allocationregion. The threads are able to access portions of the allocation regionthrough the thread structure and pointer system to identify a space inthe allocation region for the new objects. Thus, synchronization withmultiple threads through extensive use of locks may be avoided.

Similarly, promotion local allocation buffers (PLABs) may be used forobjects that are copied in the Survivor region and the Old region. Eachthread may have a PLAB in the Survivor region and the Old region whereobjects may be copied. For example, during the operations 930, 940,objects may be copied into PLABs associated with a garbage collectorthread. By use of PLABs, the number of times a lock is used duringobject copying may also be limited.

The description above includes example systems, methods, techniques,instruction sequences, and/or computer program products that embodytechniques of the present disclosure. However, it is understood that thedescribed disclosure may be practiced without these specific details.

In the present disclosure, the methods disclosed may be implemented assets of instructions or software readable by a device. Further, it isunderstood that the specific order or hierarchy of steps in the methodsdisclosed are instances of example approaches. Based upon designpreferences, it is understood that the specific order or hierarchy ofsteps in the method can be rearranged while remaining within thedisclosed subject matter. The accompanying method claims presentelements of the various steps in a sample order, and are not necessarilymeant to be limited to the specific order or hierarchy presented.

The described disclosure may be provided as a computer program product,or software, that may include a machine-readable medium having storedthereon instructions, which may be used to program a computer system (orother electronic devices) to perform a process according to the presentdisclosure. A machine-readable medium includes any mechanism for storinginformation in a form (e.g., software, processing application) readableby a machine (e.g., a computer). The machine-readable medium mayinclude, but is not limited to, magnetic storage medium (e.g., floppydiskette), optical storage medium (e.g., CD-ROM); magneto-opticalstorage medium, read only memory (ROM); random access memory (RAM);erasable programmable memory (e.g., EPROM and EEPROM); flash memory; orother types of medium suitable for storing electronic instructions.

It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes.

While the present disclosure has been described with reference tovarious embodiments, it will be understood that these embodiments areillustrative and that the scope of the disclosure is not limited tothem. Many variations, modifications, additions, and improvements arepossible. More generally, embodiments in accordance with the presentdisclosure have been described in the context of particularimplementations. Functionality may be separated or combined in blocksdifferently in various embodiments of the disclosure or described withdifferent terminology. These and other variations, modifications,additions, and improvements may fall within the scope of the disclosureas defined in the claims that follow.

What is claimed is:
 1. A non-transitory machine-readable mediumcomprising instructions, which when executed by one or more processors,cause the one or more processors to perform the following operations:identifying, by a first garbage collector thread a first object to becopied from a first memory region associated with a first locality group(lgroup) among a plurality of lgroups, to a second memory regionassociated with a second lgroup, among the plurality of lgroups, thefirst garbage collector thread associated with the second memory region;wherein the first garbage collector thread identifies the first objectby a first object identifier in a first remote queue of the firstgarbage collector thread, wherein the first garbage collector threadincludes a first local queue storing identifying information of objectsstored in the second memory region and the first remote queue storingidentifying information of objects stored in the first memory region;identifying, by the first garbage collector thread, a second object tobe copied from the second memory region, wherein the first garbagecollector thread identifies the second object by a second objectidentifier in a second remote queue of a second garbage collectorthread, the second garbage collector thread associated with the firstmemory region, responsive to determining: (a) the first object is to becopied from the first memory region, and (b) the second object is to becopied from the second memory region: prioritizing, by the first garbagecollector thread, a first operation for copying the second object fromthe second memory region above a second operation for copying the firstobject from the first memory region.
 2. The medium of claim 1, whereinthe first garbage collector thread includes a first local queue storinglocation information for objects to be copied from the second memoryregion; wherein the operations further comprise: identifying, by thefirst garbage collector thread, a third object to be copied from thesecond memory region; wherein the first garbage collector threadidentifies the third object by a third object identifier in the firstlocal queue of the first garbage collector thread, responsive toidentifying the third object based on the third object identifier in thefirst local queue: prioritizing, by the first garbage collector thread,a third operation for copying the third object above the first operationfor copying the second object.
 3. The medium of claim 1, wherein thefirst garbage collector thread includes a first local queue storingidentifying information for objects to be copied from the second memoryregion; wherein the operations further comprise: applying, by the firstgarbage collector thread, a set of rules to prioritize copyingoperations of objects by the first garbage collector, the set of rulescomprising: (1) assigning a highest priority level to copying objectsassociated with third object identifiers stored in the first localqueue; (2) assigning a second-highest level priority to copying objectsassociated with second object identifiers stored in the second remotequeue, based on the objects being located in the second memory region;(3) assigning a third-highest level priority to copying objectsassociated with first object identifiers stored in the first remotequeue, based on the objects being located in the first memory region;and (4) assigning a lowest priority to copying objects associated withfourth object identifiers stored in the second remote queue, based onthe objects being located in the first memory region.
 4. The medium ofclaim 1, wherein the operations further comprise: identifying, by thefirst garbage collector thread a third object to be copied from a thirdmemory region associated with a third locality group (lgroup) among aplurality of lgroups, the third memory region associated with a thirdgarbage collector thread; wherein the first garbage collector threadidentifies the third object by a third object identifier in a thirdremote queue, the third remote queue associated with the first garbagecollector thread, responsive to determining: (a) the third object is tobe copied from the third memory region, and (b) the second object is tobe copied from the second memory region: prioritizing, by the firstgarbage collector thread, the first operation for copying the secondobject above a third operation for copying the third object from thethird memory region.
 5. The medium of claim 4, wherein the operationsfurther comprise: obtaining, by the first garbage collector from a callstack, a fourth object identifier of a fourth object; responsive todetermining the fourth object is to be copied from the third memoryregion, storing the fourth object identifier in the third remote queue;obtaining, by the first garbage collector from the call stack, a fifthobject identifier of a fifth object; responsive to determining the fifthobject is to be copied from the first memory region, storing the fifthobject identifier in the first remote queue.
 6. A method, comprising:identifying, by a first garbage collector thread a first object to becopied from a first memory region associated with a first locality group(lgroup) among a plurality of lgroups, to a second memory regionassociated with a second lgroup, among the plurality of lgroups, thefirst garbage collector thread associated with the second memory region;wherein the first garbage collector thread identifies the first objectby a first object identifier in a first remote queue of the firstgarbage collector thread, wherein the first garbage collector threadincludes a first local queue storing identifying information of objectsstored in the second memory region and the first remote queue storingidentifying information of objects stored in the first memory region;identifying, by the first garbage collector thread, a second object tobe copied from the second memory region, wherein the first garbagecollector thread identifies the second object by a second objectidentifier in a second remote queue of a second garbage collectorthread, the second garbage collector thread associated with the firstmemory region, responsive to determining: (a) the first object is to becopied from the first memory region, and (b) the second object is to becopied from the second memory region: prioritizing, by the first garbagecollector thread, a first operation for copying the second object fromthe second memory region above a second operation for copying the firstobject from the first memory region.
 7. The method of claim 6, whereinthe first garbage collector thread includes a first local queue storingidentifying information for objects to be copied from the second memoryregion; wherein the operations further comprise: identifying, by thefirst garbage collector thread, a third object to be copied from thesecond memory region; wherein the first garbage collector threadidentifies the third object by a third object identifier in the firstlocal queue of the first garbage collector thread, responsive toidentifying the third object based on the third object identifier in thefirst local queue: prioritizing, by the first garbage collector thread,a third operation for copying the third object above the first operationfor copying the second object.
 8. The method of claim 6, wherein thefirst garbage collector thread includes a first local queue storinglocation information for objects to be copied from the second memoryregion; wherein the method further comprises: applying, by the firstgarbage collector thread, a set of rules to prioritize copyingoperations of objects by the first garbage collector, the set of rulescomprising: (1) assigning a highest priority level to copying objectsassociated with third object identifiers stored in the first localqueue; (2) assigning a second-highest level priority to copying objectsassociated with second object identifiers stored in the second remotequeue, based on the objects being located in the second memory region;(3) assigning a third-highest level priority to copying objectsassociated with first object identifiers stored in the first remotequeue, based on the objects being located in the first memory region;and (4) assigning a lowest priority to copying objects associated withfourth object identifiers stored in the second remote queue, based onthe objects being located in the first memory region.
 9. The method ofclaim 6, further comprising: identifying, by the first garbage collectorthread a third object to be copied from a third memory region associatedwith a third locality group (lgroup) among a plurality of lgroups, thethird memory region associated with a third garbage collector thread;wherein the first garbage collector thread identifies the third objectby a third object identifier in a third remote queue, the third remotequeue associated with the first garbage collector thread, responsive todetermining: (a) the third object is to be copied from the third memoryregion, and (b) the second object is to be copied from the second memoryregion: prioritizing, by the first garbage collector thread, the firstoperation for copying the second object above a third operation forcopying the third object from the third memory region.
 10. The method ofclaim 9, further comprising: obtaining, by the first garbage collectorfrom a call stack, a fourth object identifier of a fourth object;responsive to determining the fourth object is to be copied from thethird memory region, storing the fourth object identifier in the thirdremote queue; obtaining, by the first garbage collector from the callstack, a fifth object identifier of a fifth object; responsive todetermining the fifth object is to be copied from the first memoryregion, storing the fifth object identifier in the first remote queue.11. A system, comprising: one or more processors; and memory storinginstructions that, when executed by the one or more processors, causethe system to perform: identifying, by a first garbage collector threada first object to be copied from a first memory region associated with afirst locality group (lgroup) among a plurality of lgroups, to a secondmemory region associated with a second lgroup, among the plurality oflgroups, the first garbage collector thread associated with the secondmemory region; wherein the first garbage collector thread identifies thefirst object by a first object identifier in a first remote queue of thefirst garbage collector thread, wherein the first garbage collectorthread includes a first local queue storing identifying information ofobjects stored in the second memory region and the first remote queuestoring identifying information of objects stored in the first memoryregion; identifying, by the first garbage collector thread, a secondobject to be copied from the second memory region, wherein the firstgarbage collector thread identifies the second object by a second objectidentifier in a second remote queue of a second garbage collectorthread, the second garbage collector thread associated with the firstmemory region, responsive to determining: (a) the first object is to becopied from the first memory region, and (b) the second object is to becopied from the second memory region: prioritizing, by the first garbagecollector thread, a first operation for copying the second object fromthe second memory region above a second operation for copying the firstobject from the first memory region.
 12. The system of claim 11, whereinthe first garbage collector thread includes a first local queue storingidentifying information for objects to be copied from the second memoryregion; wherein the operations further comprise: identifying, by thefirst garbage collector thread, a third object to be copied from thesecond memory region; wherein the first garbage collector threadidentifies the third object by a third object identifier in the firstlocal queue of the first garbage collector thread, responsive toidentifying the third object based on the third object identifier in thefirst local queue: prioritizing, by the first garbage collector thread,a third operation for copying the third object above the first operationfor copying the second object.
 13. The system of claim 11, wherein thefirst garbage collector thread includes a first local queue storinglocation information for objects to be copied from the second memoryregion; wherein the instructions further cause the system to perform:applying, by the first garbage collector thread, a set of rules toprioritize copying operations of objects by the first garbage collector,the set of rules comprising: (1) assigning a highest priority level tocopying objects associated with third object identifiers stored in thefirst local queue; (2) assigning a second-highest level priority tocopying objects associated with second object identifiers stored in thesecond remote queue, based on the objects being located in the secondmemory region; (3) assigning a third-highest level priority to copyingobjects associated with first object identifiers stored in the firstremote queue, based on the objects being located in the first memoryregion; and (4) assigning a lowest priority to copying objectsassociated with fourth object identifiers stored in the second remotequeue, based on the objects being located in the first memory region.14. The system of claim 11, wherein the instructions further cause thesystem to perform: identifying, by the first garbage collector thread athird object to be copied from a third memory region associated with athird locality group (lgroup) among a plurality of lgroups, the thirdmemory region associated with a third garbage collector thread; whereinthe first garbage collector thread identifies the third object by athird object identifier in a third remote queue, the third remote queueassociated with the first garbage collector thread, responsive todetermining: (a) the third object is to be copied from the third memoryregion, and (b) the second object is to be copied from the second memoryregion: prioritizing, by the first garbage collector thread, the firstoperation for copying the second object above a third operation forcopying the third object from the third memory region.
 15. The system ofclaim 14, wherein the instructions further cause the system to perform:obtaining, by the first garbage collector from a call stack, a fourthobject identifier of a fourth object; responsive to determining thefourth object is to be copied from the third memory region, storing thefourth object identifier in the third remote queue; obtaining, by thefirst garbage collector from the call stack, a fifth object identifierof a fifth object; responsive to determining the fifth object is to becopied from the first memory region, storing the fifth object identifierin the first remote queue.